The present invention relates to an apparatus for accomplishing the gas coating of particles suspended in a fluidized bed and more particularly to such an apparatus where the particles are dispersed and suspended in the fluidized bed by means of a levitating gas with reactant or coating gas being introduced into the fluidized bed through an elongated nozzle extending upwardly from the coating chamber floor.
It is well known in the prior art to employ gas coating within a fluidized bed for coating particles and other objects immersed in the bed. The coating may be formed by the decomposition, polymerization, condensation and/or chemical reaction of a gas, vapor or gas-vapor mixture. As a particular example of such a process, pyrolytic carbon or metallic carbides are commonly applied by such a process to provide protection for nuclear fuel particles of the type used in nuclear reactors. The fuel particles are small, for example, on the order of 500 microns, and may be formed from a suitable fissle and/or fertile material such as uranium, plutonium, thorium or suitable compounds of such materials.
Within a nuclear reactor, the nuclear fuel particles are exposed to conditions of high temperature and severe irradiation over long periods of operation. In order to assure continued effectiveness within such an environment, it has become common practice to coat the fuel particles with an impermeable material in order to retain gaseous and metallic fission products within the confines of the individual particles.
The coatings may be applied within a high temperature coating chamber through the introduction of a reactant gas having as a substantial component, or consisting entirely of, a suitable hydrocarbon such as acetylene, propylene, propane or methane. The desired coating is thus deposited through the high temperature decomposition of the hydrocarbon gas.
Examples of fuel particles provided with such coatings are disclosed and set forth for example in U.S. Pat. No. 3,325,363 issued June 13, 1967 to Goeddel et al.; U.S. Pat. No. 3,298,921 issued Jan. 17, 1968 to Bokros et al.; U.S. Pat. No. 3,361,638 issued Jan. 2, 1968 to Bokros et al.; and U.S. Pat. No. 3,649,452, issued Mar. 14, 1972 to Chin et al.
When the particles to be coated are relatively small, the coating operation may be efficiently carried out with the particles suspended in a fluidized bed within a high temperature coating chamber. The levitation or suspension of the particles within the fluidized bed may be achieved through the controlled introduction of a hydrocarbon gas, an inert carrier gas or a combination thereof into the coating chamber for dispersion through the particle bed. Most commonly, an inert carrier gas such as argon, helium, nitrogen or hydrogen is employed for this purpose.
In a coating chamber for such an operation, the particles may be dispersed and suspended by means of the inert gas in order to form a fluidized bed. A reactant gas may be introduced into the fluidized bed by means of a nozzle, the reactant gas being decomposed by high temperature within the coating chamber in order to deposit the coated material upon the particles. The various conditions for carrying out such a coating operation are well known in the prior art including temperature ranges for the coating chamber as well as rates and pressures under which both the reactive and levitating gases may be introduced as well as the duration of the coating operation.
High temperature gas coating operations of this type are relatively complex and may accordingly involve numerous problem areas which must be resolved in order to deposit uniform coatings throughout prolonged operations. Two such problem areas are particularly contemplated in connection with the present invention.
Initially, a problem arises due to introduction of the reactant gas into the high temperature coating chamber through a vertically extending nozzle which is open at its other end. A buildup of carbonaceous material, commonly referred to as "overcoating", may occur at the open end of the nozzle. This phenomenon is a particular problem since the carbonaceous overcoating on the nozzle interferes with the rate of flow for the reactant gas entering the coating chamber and also interferes with the pattern of dispersion for the reactant gas throughout the coating chamber.
An additional problem area of particular concern in relation to the present invention involves the need for maintaining uniform distribution and circulation of particles throughout the fluidized bed. These factors of uniform distribution and particle circulation are particularly important in developing uniform coatings for entire particle batches in the coating chamber.
Accordingly, there has been found to remain a substantial need for an improved method and apparatus for carrying out the gas coating of particles suspended in a fluidized bed.